1. Field of the Invention
This invention relates generally to a magnetic recording device in which there is a rotating magnetic disk on which servo information has been written to guide the positioning of a read/write transducer. More particularly, it relates to such a device wherein the use of the servo information has been optimized to cancel the effects of repeatable runout (RRO).
2. Description of the Related Art
As shown schematically in FIG. 1, a typical magnetic disk (1) mounted on a rotatable spindle (12) in a hard disk drive (HDD) (11) is characterized by radially concentric, annular, circular tracks (17) on which data can be written and from which data is read by means of a read/write transducer (13) mounted on an actuator assembly (14). In order to for the read/write transducer to accurately follow such a circular track while executing reading and writing operations, i.e., to maintain a fixed radial position over the centerline of the track as the disk rotates, it is necessary to be able to position the head precisely at various places on the disk. Such positioning is typically accomplished by means of a closed loop servomechanism (not shown), which is a mechanism that accepts an input signal indicating an actual position of the transducer over a track, then determines the position at which the transducer should actually be positioned (typically the radial centerline of the track) and feeds back a correction related to the difference between the intended and actual positions which can be used to reposition the transducer. The signal produced by the difference between the intended and actual positions is called a position error signal, PES and, since it is typically digital in nature, it is processed by a DSP to compute a correction signal which is fed into a digital to analog converter (DAC) which then sends a corresponding analog signal to a voice coil activator (VCA) that finally repositions the transducer as necessary. The mechanism by which such a PES is obtained is through the use of imprinted data on the disk, called servo data. This servo data is recorded on periodically repeated, small angular wedges (16) located within each annular track. Two such wedges are shown here and, for simplicity, they are indicated as rectangular in shape. The servo data is in the form of “bursts” of magnetic transitions (small changes in magnetization) that are typically both radially and angularly separated from each other within each wedge. As the disk rotates and the transducer passes over each such wedge, also referred to as a servo sector, it reads its position relative to the bursts and can tell if it is not equidistant between them, indicating a displacement from the centerline of the track. The distance between the transducer and the track centerline is the PES and it is that signal that is fed back to the servo mechanism for purposes of correcting the transducer's position. The servo controller, which determines the PES, calculates a correction value and sends it to the DAC, has a finite, frequency dependent, position error correcting capability. Thus, if the signal sent from the transducer indicating its deviation from the centerline is a complicated oscillatory waveform (as a function of deviation vs. servo wedge location), the closed loop servo mechanism will be unable to adequately correct the misalignment of the transducer. One approach to avoiding the necessity of the servo mechanism having to track a complicated oscillation, is to remove regular components of that oscillation if possible. As we shall discuss below, the oscillations that the servo mechanism is forced to follow are generally composed of two components: 1. a component that is regularly repeated at some oscillation frequency or combination of frequencies, related to the rotational frequency of the disk, and 2. a component that is random and has no regularity to it. The first such component is called repeatable runout (RRO) and the second is called non-repeatable runout (NRRO).
Ideally, each annular track should be concentric with the drive spindle about which the disk is rotating and should remain so during HDD operation. If this is the case, the PES will be zero and the closed loop servo mechanism will make no correction to the transducer's location. In practice, however, the tracks will not be concentric and corrections will have to be made. There are many reasons for this lack of concentricity. One reason of importance is a lack of concentricity of the track with the spindle or failure of the track to be circular to begin with. These will lead to the track having an eccentric motion about the spindle during disk rotation.
Referring to FIG. 2, there is shown a hard disk (1) whose center is nominally at the center of the spindle opening (12). Because of some unspecified rotational deviation, the disk is actually rotating about a displaced center (14). As a result, the transducer is tracing out the broken line (15) when it should be tracking the originally concentric tracks (17). As the transducer's broken line trajectory passes over servo wedges (16) at points of intersection (16a), (16b), etc., the servo mechanism will try to move the transducer back into alignment with the tracks (17). An observation of the transducer, for a period of time, at a fixed point in space will show an oscillatory motion. Other reasons for such transducer oscillations include factors such as disk slippage, disk warpage and even poorly written servo data. Although the effect of track eccentricity is problematic, it has the virtue of being periodic and, as noted above, it is designated repeatable runout (RRO).
RRO at any point on the disk within a circular track can be defined as the motion of that point relative to another point that is fixed in space. The problem caused by RRO is due to the fact that the HDD transducer is trying to follow the motion of that point. The periodicity of RRO allows for the possibility of its elimination by any of several means, some of which will be discussed below. Another source of positional error is called non-repeatable runout (NRRO), which is caused by various random and environmental effects on the motion of the disk and which is not easily eliminated. Clearly, however, if the effects of RRO can be significantly diminished, then it is easier for the servo mechanism to correct for the NRRO.
If the RRO is slight, the servo mechanism can compensate for it by use of the closed loop servo mechanism within the HDD. However, if the RRO exceeds some predetermined tolerance defined by the manufacturing parameters of the disk drive, then the servo mechanism is incapable of completely correcting for it and the disk drive will not operate properly.
One approach to compensating for RRO is to predetermine the amount of RRO present at each servo sector and feed that information, once and for all, into the servo mechanism in advance of the drive use. This is done by inputting data into an array that stores data relevant to where the transducer will be when it reaches the location of servo wedge i+1, based on a calculation done at the position of servo wedge i. Thus, when the transducer is over wedge i, is corrected for where it is about to be positioned. This is called a static feed forward compensation. Alternatively, this feed forward compensation can occur at various times during the drive use, which is called adaptive feed forward compensation.
By feeding these compensated values into the servo mechanism (eg., storing them in an array), the servo mechanism will regard those stored values of RRO as being normal, so to speak, and will not try to “correct” them. Of course, any amount of NRRO that occurs during drive operation will now be determined relative to this stored RRO and the PES for that additional misalignment will be acted upon by the servo mechanism.
Determining the best PES values to feed forward into the system to compensate for RRO is not a simple matter. If the system is simply run and measurements of PES values are taken at selected points along each track, the PES values measured by the system will include not just the RRO but all other misalignments as well. One way of determining the RRO effects while ignoring other perturbing misalignments, is to determine a waveform for the PES as a function of disk angle of rotation over several rotations, and then average their results. Since NRRO is typically random in nature, the averaging will tend to eliminate their effects. Once the average oscillation waveform is determined, an analysis of the harmonic content of this waveform can be done. Such an harmonic analysis will display the RRO component as various multiples of the disk rotational frequency. When doing the rotation averages prior to such a harmonic analysis, it is advantageous to eliminate as much of the NRRO as possible by setting the servo mechanism at a low bandwidth condition, so that the actuator mounted transducer will have a lowered sensitivity to random effects. The present inventor, in Drouin (U.S. Pat. No. 5,550,685), which is incorporated herein in its entirety by reference, described such a method for compensation of RRO, using a Fourier transform and back transform to identify and compensate the effects of various frequency components of the RRO waveform. This method required an identification of the particular frequencies to be compensated, required an extensive computation for each frequency identified and required the application of a Fourier transform algorithm. As compared to the method of the present invention to be described below, this previous method was computationally intensive, utilized much storage capacity and was time consuming. As is briefly described below, however, there is much additional prior art to be found that describes other forms of such algorithms and methods to apply them.
Smith et al. (U.S. Pat. No. 6,700,728) shows a feed forward system using the average of PES values. Smith is particularly concerned with correcting for PES outliers, whose extreme values can adversely affect attempts to compensate for RRO.
Cho et al. (U.S. Pat. No. 7,042,827) discloses a method of calculating feed forward values by running a disk drive at a variety of speeds.
Cunningham et al. (U.S. Pat. No. 5,854,722) teaches feed forward correction signals between servo sectors. More specifically, however, Cunningham is concerned with a method for compensating for effects of the arc-like path of the actuator arm as it tracks along the surface of the disk.
Melkote et al. (U.S. Pat. No. 6,999,267) describes RRO compensation iteratively learned for each servo sector using the previously learned value for each sector and PES for each sector and adjacent sectors.
Melkote et al. (U.S. Pat. No. 6,826,006) discloses a method of calculating RRO cancellation values base on values for each servo wedge.
Melkote et al. (U.S. Pat. No. 6,924,959) shows a method of estimating RRO values based on current PES values and a previous estimation of RRO errors.
Yi et al. (U.S. Pat. No. 7,196,864) describes a first servo-loop compensator that processes PES values during RRO calibration.
Li et al. (U.S. Pat. No. 7,286,317) teaches compensation for RRO by measuring timing between information read on the disk.
None of this prior art achieves the ends desired in the present invention, namely an accurate, simple and computationally less time and storage-space consuming method to eliminate the effects of RRO.